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This is because gestures are more likely to physically reflect what they mean, a property known as iconicity, says linguist Simon Garrod at the University of Glasgow. Although there are cases of words that mimic their meaning – drip, growl, splash, for example – these are few and far between. For most words, the connection between their sound and their meaning is fairly arbitrary.

Marcus Perlman from the University of Wisconsin-Madison and his colleagues wanted to put this assumption to the test. They asked nine pairs of students to play an elaborate game of vocal charades, in which they had to express certain words, such as big, slow or attractive, using only simple vocalisations. No gestures or facial expressions were allowed.

From the outset, the students tended to pick vocalisations with similar acoustic properties, such as duration and pitch, for many of the words. Over time, as these words were said back and forth they became increasingly similar, both within pairs, and between. As an example, here are recreations of the vocalisations people made to represent big, small, rough and smooth.

Meaningful grunts

Ten of the vocalisations were then played back to a new group of people. They were able to match the vocalisation with its assigned meaning from a list of 10 words on average 35 per cent of the time, much higher than chance.

Perlman reasons that this is because these vocalisations are indeed iconic – in the minds of the students, their acoustic properties somehow resemble the meaning they are trying to convey. So it’s not infeasible that our ancestors’ grunts may have had meaning.

Garrod says the results support the idea that iconicity is key in developing ways to communicate – “gestural or otherwise”.

He believes that spoken language may have evolved in a similar way to sign language. This starts out with iconic hand movements that gradually become replaced with more abstract ones. “If the experiment were run for longer, I would expect that the vocalisations would become increasingly arbitrary,” says Perlman.